One of conventional semiconductor lasers has been described in detail on pages 195 to 202 of "JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. LT-1, No. 1, MARCH 1983". The semiconductor laser is of a type of Double Channel Planer Buried Heterostructure Laser Diode as simply called "DC-PBH-LD" which is fabricated, for instance by a two-step iiquid phase epitaxy growth process. That is to say, a double heterostructure wafer is grown to have an n-InP buffer layer, an InGaAsP active layer, and a p-InP cladding layer on a &lt;001&gt;n-InP substrate in the first step thereby forming mesa stripe of about 2 .mu.m-width in the &lt;110&gt; direction in accordance with a pair of 7 .mu.m-wide and 3 .mu.m-deep channels by using Br-methanol solution and photoresist mask. In the second step, a p-InP current blocking layer and an n-InP current confining layer are grown exclusively on the channels and the remaining flat parts excluding the top of the mesa, and a p-InP embedding layer and a p-InGaAsP cap layer are successively grown thereon to provide a substantially flat surfaced DC-PBH-LD.
As a result, there is fabricated a semiconductor laser which comprises buried layers having pnpn junction structures positioned at both sides of a double heterostructure including an active region.
In operation of the semiconductor laser, np reverse junctions of the pnpn junction structures prevent current from flowing thereby resulting in selective current injection into the active region.
In a current blocking structure including such an np reverse-junction, the current blocking can be effectively made so that the quantum efficiency of more than 50% is obtained.
According to the conventional semiconductor laser, however, the np reverse-junction presents the capacitance in its value exceeding 10 pF so that the high speed modulation is difficult to be performed at a rate of more than 4Gb/s for the reason why the time constant thereof becomes larger.
There has been fabricated another conventional buried heterostructure semiconductor laser as described on pages 856 to 857 of "ELECTRONICS LETTERS,VOL. 20, OCT. 11, 1984" in which an active region is buried at both sides thereof by intrinsic semiconductors (called "i-layer" hereinafter).
In operation of the semiconductor laser, the high speed modulation can be performed at a rate of more than 4Gb/s for the reason why a pn junction which suppresses the high speed modulation is only provided in the active region. On the other hand, it is designed to reduce the leakage current flowing outside the active region in accordance with the high resistance of the i-layer.
According to the second conventional semiconductor laser, however, the quantum efficiency is not highly obtained due to the fact that the resistance of the i-layer does not practically become so large as expected. In more detail, there is a limitation that the concentration of an impurity does not become lower than about 1.10.sup.14 cm.sup.-3 in regard to background impurity level for physical reasons in a case where the i-layer is formed in a practical epitaxial growth. In such a case, the resistivity of the InP i-layer is about 10 106 -cm so that it is difficult to provide a sufficient current blocking effect.
There has been fabricated still another conventional buried heterostructure semiconductor laser in which an active region is buried at both sides thereof by insulating layers as described in detail on pages 78 to 100 of "APPLIED PHYSICS LETTERS, VOL. 47, JULY 15, 1985", which was published after the Convention date of the instant application.
In operation of the semiconductor laser, the high speed modulation can be performed at a rate of more than 4Gb/s for the same reason as the second conventional semiconductor laser. At the same time, the current blocking is effectively achieved due to the high resistance of the insulating layers.
According to the third conventional semiconductor laser, however, the reliability thereof is not highly obtained for the reason why the active region is in contact at both sides thereof with the insulating layers so that defects tend to occur in the active region.
As described above, the high speed modulation, quantum efficiency, and reliability are difficult to be achieved as expected in the respective conventional buried heterostructure semiconductor lasers.